The invention relates to a transmission unit according to the preamble of claim 1.
With drives, it may be necessary for various reasons to arrange a shift transmission between the drive motor and the consuming device. One reason for this may be that the speeds of rotation desired for the consuming device differ from the output speed of rotation of a given drive motor to such an extent that a shift transmission must be provided. A further reason may be that with certain consuming devices, such as for example vehicles and more particular construction-site vehicles, particular wheel and chain drives are pre-provided, which have to be taken into consideration when selecting the drive motor. A further basic reason for the use of a shift transmission is that a shift transmission makes possible the functioning of the available drive in speed of rotation ranges favourable for the drive motor, which is in particular the case when using combustion engines, preferably Diesel engines, as drive motors. In this case it must be emphasized that in particular construction-site vehicles such as wheeled loaders, earth moving machines, excavators and the like, as well as municipal tractors and equipment carriers, are preferably driven with combustion engines, in particular Diesel engines, whereby a drive which is independent of a fixed-location source of energy is provided, which is powerful and which can be economically operated.
In particular with the aforementioned construction-site vehicles and apparatus it is likewise desired to be able to shift gear in substance without load or drive interruption, which to a large extent can be realized by means of so-called powershift transmissions.
A transmission unit of the kind as mentioned above is described in DE 34 33 495 C2 for semi-automatic gear shifting and in DE 34 33 494 C2 for automatic shifting. Each of these two transmission units comprises an adjustable hydrostatic transmission and a powershift transmission, whereby in all cases the adjustability of the hydrostatic transmission is utilized on the one hand to regulate the transmitted power by means of a power regulator and on the other hand to bring about a matching of speed of rotation, determined by the gear to be selected, for a gear shifting operation. The matching of speed of rotation occurs in the region of a coupling at the output of the hydrostatic transmission (DE 34 33 495 C2) or in the region of the coupling of the respective powershift gear (DE 34 33 495 C2), in which case the output speed of rotation of the hydrostatic transmission is adjusted in such a way that there exists speed parity at the relevant coupling, whereafter gear shifting is performed without jerking. In doing this, an alteration of the speed of rotation of the drive motor is not necessarily required, which motor is in these known configurations constituted by a combustion engine.
With the known transmission units there is great outlay inevitably involved in terms of design and also technically for the shifting. The latter results from the fact that, for a gear shift, the output speed of rotation of the hydrostatic transmission must be purposively matched to the necessary speed of rotation for the gear step. Furthermore, through this purposive matching, a considerable time is required for shifting gears. A further disadvantage with the known transmission units is based on the fact that powershift transmissions usually have multi-disc clutches, which not only are comparatively expensive to manufacture but also require a complex control and regulation technique and moreover generate a comparatively large amount of heat, which results in an unfavourable thermal balance. With the known transmission units, the efficiency and power transfer are therefore unsatisfactory.